It must be at least 22mm pipe.
15mm from the meter has never been acceptable, regs stated 22mm to within 1 metre of appliance.
Basically you have know about pipes, sizes, appliance gas rates etc – it’s not as simple as just connecting up.
If incorrect pipe size is used – if you have a gas fire, oven or hob and the boiler comes on, the other appliances could be starved of gas. When the boiler goes off – the gas is back at hob/fire etc but not lit – extremely dangerous.
22mm is the minimum size of any gas pipe from the meter, longer runs/more appliances 28mm and so on